Default is still high speed, but setting
SPEED=high|full work as expected.
`make BOARD=same70_xplained SPEED=full`
Tested with examples/device/cdc_msc
```preformatted
lsusb -v
...
TinyUSB Device:
Product ID: 0x4003
Vendor ID: 0xcafe
Version: 1.00
Serial Number: 0123456789ABCDEF
Speed: Up to 12 Mb/s <------ full speed here
Manufacturer: TinyUSB
Location ID: 0x03142130 / 12
Current Available (mA): 500
Current Required (mA): 100
Extra Operating Current (mA): 0
Media:
Mass Storage:
Capacity: 8 KB (8,192 bytes)
Removable Media: Yes
BSD Name: disk5
Logical Unit: 0
Partition Map Type: Unknown
S.M.A.R.T. status: Verified
USB Interface: 2
```